If you have more than 50 posts, you may also specify your own custom avatar.

Then click User CP tab above>> Edit Options>> Scroll down to lower part of the page to Other Options>> Click Change Avataer tab. Once you are brought into the new page for avatars, scroll down to Custom Avatars then click yes. If you know the url of the image you want to copy and paste on the space provided or you could upload an image from your own computer.